This invention relates to improving the radiation pattern of ultrasonic transducers.
In recent years ultrasonic imaging techniques have become prevalent in clinical medical diagnosis. Such techniques have been utilized for some time in the fields of obstetrics, neurology, and cardiography. This technique has been used to measure and record the dimensions and position of deep lying organs and physiological structures throughout the body. A wide variety of ultrasound transducers and systems are used for imaging purposes. The systems range from a single crystal mechanically swept scanner, to linear arrays, to phased array sector scanners.
Phased array ultrasound imaging systems have gained wide acceptance as the primary method of ultrasound imaging, particularly due to an ability to electronically form, focus, and steer an ultrasound imaging beam in the imaging plane. Ideally, the result is a thin beam of ultrasound energy which can be steered in a lateral direction to provide an imaging plane. Typically, a plurality of parallel piezoelectric transducer elements are arranged as parallel columns along the lateral direction of the transducer to form a phased array transducer, with beamforming and steering control in the lateral direction. Controlling the ultrasound beamforming in the elevational plane is more difficult since typically there are no multiple transducer elements in the elevational direction with which to electronically focus the beam. An acoustic lens placed in front of the transducer is often used to obtain a single elevational focus for the generated ultrasound beam. However, diffraction due to the finite length of the transducer crystal in the elevational direction causes side lobes to appear in elevation which interfere with imaging by the main lobe.
Apodization, application of an acoustic amplitude weighted window across the transducer crystal in the elevational direction, has been shown to reduce the level of elevational side lobes. Apodization methods which have been used to control elevational side lobes include selectively poling the transducer crystal to modulate the polarization efficiency of the piezoelectric crystal in the elevational direction, and electrode shading in which an acoustic attenuative material of varying thickness is overlaid on the edge of the transducer crystal to attenuate the output of the crystal as a function of the material thickness.